1. Field of the Invention
The present invention relates to: an image bearing member such that abrasion resistance and scratch resistance are high, and crack or film peeling is hard to occur, and that the image bearing member comprises a photoconductive layer having favorable electrical properties; and an image forming apparatus, a process cartridge and an image forming method that are used for a color photocopier, color printer, etc.
2. Description of the Related Art
In recent years, in order to write digital signal data, a method for forming an electrophotographic image has increasingly been employed in which a latent electrostatic image is formed on an organic photoconductor by dot light exposure and developed by a reversal developing method to form an image.
It is required that organic photoconductors employed in such a method be stable over long term use and also suitable for high resolution writing; however, the strength of organic photoconductors is low and problems due to abrasion wear of and scratches on a photoconductive layer are likely to occur. Thus, improvement of durability has been sought for.
As a technique to improve the abrasion resistance of the photoconductive layer, for example, (1) a photoconductor in which a hardening binder is used in a crosslinked charge transport layer (see Japanese Patent Application Laid-Open (JP-A) No. 56-48637), (2) a photoconductor in which a polymeric charge transport material is used (see JP-A No. 64-1728), and (3) a photoconductor in which an inorganic filler is dispersed in a crosslinked charge transport layer (see JP-A No. 4-281461) are proposed.
Among these techniques, in case of the photoconductor of (1) using a hardening binder, poor compatibility with a charge transport material and impurities such as a polymerization initiator and an unreacted residual group increase a rest potential, inviting the decrease of image density. In case of the photoconductor of (2) using a polymeric charge transport material, abrasion resistance has been improved to some extent, however, the photoconductor of (2) does not have satisfactory durability sought for organic photoconductors. In addition, the electrical properties of polymeric charge transport material are hard to be stable since the polymerization of polymeric charge transport material and its purification is difficult, and thus it is difficult to obtain charge transport polymer material with high purity. Further, there are problems during manufacture such as high viscosity of coating solution. The photoconductor of (3) in which an inorganic filler is dispersed in a crosslinked charge transport layer has higher abrasion resistance compared to the common photoconductor in which low-molecular-mass charge transport material is dispersed in an inactive polymer; however, a charge trap present on the surface of the inorganic filler increases a rest potential, inviting the decrease of image density. In addition, when the inorganic filler and a binder resin form large concavities and convexities on the surface of the photoconductor, cleaning failures may occur, causing toner filming and image deletion. These techniques of (1), (2) and (3) have not satisfied an overall durability including electrical durability and mechanical durability required for an organic photoconductor yet.
Further, in order to improve the abrasion resistance and the scratch resistance of the photoconductor of (1), Japanese Patent (JP-B) No. 3262488 proposes a photoconductor in which a cured material of a multi-functional acrylate monomer is included. In this patent literature, it is described that a cured material of a multi-functional acrylate monomer is included into a protective layer on a photoconductive layer; however, the literature only describes that a charge transport material may be included into the protective layer and there exist no specific descriptions. In addition, when simply adding a low-molecular-mass charge transport material to a crosslinked charge transport layer, the problem of compatibility with the cured material occurs. As a result, the low-molecular-mass charge transport material separates out and a white turbidity appears. The increase of the electric potential at an exposed area caused not only the reduction of image density, but also the reduction of mechanical strength in some cases. Also, the proposed photoconductor is produced, specifically, by way of causing monomers contained in a reaction mixture together with a polymer binder to react; therefore, a three-dimensional network structure is not fully developed, and a crosslink density becomes low. Thus, the photoconductor is not so satisfactory as to exert noticeable abrasion resistance.
As a technique for improving the abrasion resistance of photoconductive layer in place of these techniques, JP-B No. 3194392 proposes to provide a charge transport layer formed using a coating solution that comprises a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond, and a binder resin. The binder resin is considered to improve adhesion of the charge generating layer and a cured charge transport layer, and further to have a role to ease the internal stress of a film when a thick film is cured. The binder resin is broadly classified into a binder that has a carbon-carbon double bond and is reactive with the charge transport material and another binder that does not have the double bond and is non-reactive with the charge transport material. The proposed photoconductor represents both abrasion resistance and favorable electrical properties, which attracts attention. However, when a binder resin not having a reactivity with a charge transport material is used, the non-reactive resin is not well compatible with the cured material generated from the reaction between the monomer and the charge transport material, and phase separation is likely to occur in the crosslinked charge transport layer, which may cause scratches, fixation of an external additive in the toner and paper dusts. Further, as mentioned above, three-dimensional network structure is not fully developed, and a crosslink density becomes low. Thus, the photoconductor is not so satisfactory as to exert noticeable abrasion resistance. In addition, specifically described monomers for use in this photoconductor are bifunctional. From these reasons, the photoconductor was not satisfactory in terms of abrasion resistance. Also, even when the binder resin having a reactivity is used, the number of crosslinkage between molecules is small although the molecular mass of the cured material increases. It is difficult to obtain both proper binding amount of the charge transport material and proper crosslink density at the same time, and thus electric properties and abrasion resistance were not satisfactory.
Further, JP-A No. 2000-66425 discloses a photoconductor that comprises a photoconductive layer containing a cured compound of a hole transporting compound having two or more functional groups capable of undergoing chain polymerization in the same molecule. This photoconductive layer has a high hardness due to increased crosslink density. However, since the bulky hole transporting compound has two or more functional groups capable of undergoing chain polymerization, distortion occurs in the cured material and internal stress increases, and crack or peeling tends to occur in the crosslinked surface layer during long-term use in some cases.
Further, in order to improve the abrasion resistance, a photoconductor has been put to practical use that is provided with a photoconductive layer or a surface protective layer that comprises an organosilicon binder resin with high durability. However, the organosilicon binder resin easily absorbs moisture, causing problems such as reduced quality of images, specifically image blur and deletion due to filming. Further, in the crosslinked film of organosilicon, an unreacted hydrolyzable group and silanol group remain on the surface of the film easily, and thus the crosslinked film of organosilicon has a drawback that it is easily affected to the adsorption of water molecules under a high-humidity environment. Much unreacted group causes easily adsorption of water molecules and discharge products generated upon charging under a high-humidity environment. As a result, surface resistance is reduced, causing problems such as image deletion.
As one of measures to such image deletion caused by moisture absorption, it is known that a photoconductor is provided with a heating device and heated therewith (see JP-A No. 2000-241998).
Heating of photoconductor by providing the heating device can prevent image deletion when images are being formed. In this case, however, toner filming is likely to occur. For example, there is a drawback that image deletion occurs the day after the stop of image forming apparatus due to the moisture absorption by a filming substance. The higher durability and the higher abrasion resistance the photoconductor has, the smaller the abrasion loss of the surface layer is, causing the deterioration of the surface, which occurs during charging, or making the removal of charge products difficult. Consequently, image deletion occurs or dot reproducibility deteriorates. Especially, these phenomena are observed remarkably at the site closely near the charging electrode during the halt of a photoconductor drum. For example, it is difficult to suppress image deletion phenomenon occurring beneath a charging electrode sufficiently by airflow or the heating device arranged closely near the photoconductor drum. It is probably because even after the halt of the operation of image forming apparatus, harmful materials such as active oxygen generated during operation remain closely near each charging electrode and act on the coated photoconductive layer at the surface of the photoconductor after the halt of rotation. In addition, it is impossible to heat the surface of the photoconductor uniformly by conventional means such as blowing of airflow and a heating device arranged closely near the photoconductor as a separate body, and thus such conventional means are insufficient for preventing e.g. adsorption of water molecules under a high-humidity environment.
There has not been provided an image bearing member such that the image bearing member has a favorable charging properties and has high sensitivity and high abrasion resistance, the surface resistance is not reduced even under a high-humidity environment, and problems such as image deletion are not caused, and that even after the formed image has been left, image deletion is not caused and the image quality is remarkably stable. Thus, in the present situation, it has been highly desired to develop such image bearing member as soon as possible.